This application is based on Japanese Patent Application No. 2001-7404 filed on Jan. 16, 2001 and No. 2001-330739 filed on Oct. 29, 2001 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an emission control apparatus for an engine and a method for reducing emissions of an engine.
2. Description of Related Art
JP-A-2000-2138 discloses an apparatus for reducing emissions of an engine that has series disposed two HC absorbent catalysts. The HC absorbent catalyst (HC catalyst) carries a hydrocarbon (HC) absorbent (e.g. zeolite) for absorbing HC when the HC catalyst is at a relatively low temperature (absorbing condition) such as just after the engine start and for desorbing the absorbed HC when the HC catalyst is heated to a predetermined temperature range (desorbing condition). The HC catalyst may carry three-way catalyst components for reducing emissions when the HC catalyst is heated to an activated temperature range (activated condition).
JP-A-2000-2138 also discloses a feedback controller for maintaining air-fuel ratio (A/F) of an exhaust gas in the HC catalyst at a stoichiometric value when the catalyst reaches the activated condition. This air-fuel ratio control may reduce emissions when both the catalysts are in the activated condition. However, the HC catalyst disposed downstream may not oxidize desorbed HG since both the HC catalysts may not be activated simultaneously. For example, the air-fuel ratio in the HC catalyst disposed downstream may exceed too rich, since the HC catalyst disposed downstream still in the desorbing condition when the HC catalyst disposed upstream reaches the activated condition.
The present invention addresses these drawbacks by providing an emission control apparatus for an engine and a method for reducing emissions of an engine that are capable of reducing an amount of HC emissions.
According to the present invention, a HC absorbent catalyst desorbs absorbed HC when the HC absorbent catalyst is in a desorbing condition. An air-fuel ratio of exhaust gas in the HC absorbent catalyst is controlled by at least one of feedback loops. At least one of target values associated with a HC absorbent catalyst by one of feedback loops is varied to a leaner value when the HC absorbent catalyst is in a desorbing condition. The leaner target value provides a leaner atmosphere in the HC catalyst, and provides sufficient oxygen to oxidize HC desorbed from the HC absorbent catalyst itself.
For example, in the case that a first catalyst disposed upstream is the HC absorbent catalyst, a first target value for an upstream of the first catalyst may be corrected to the leaner value, since the first target value is associated with the HC absorbent catalyst by a main feedback loop. In the case that a second catalyst disposed downstream is the HC absorbent catalyst, a second target value for an upstream of the second catalyst may be varied to the leaner value, since the second target value is associated with the HC absorbent catalyst by a first sub-feedback loop. Further, in the case that both of the first and second catalysts are the HC absorbent catalysts, at least one of the target value or both of the first and second target values may be varied to the leaner values.
In the present invention, whether the HC absorbent catalyst is in the desorbing condition or not may be determined by detecting a temperature of the HC absorbent catalyst. For example, the desorbing condition may be detected when the temperature reaches the predetermined temperature range. The temperature may be detected or presumed from the HC absorbent catalyst, the exhaust gas, elapsed time from starting the engine or the like. In the case of using the elapsed time, the target value may be varied when the elapsed time reaches a fixed threshold value. Additionally, the threshold value may be compensated based on a factor that affects a period of time to heat the HC absorbent catalyst to the predetermined temperature range from the engine start. For example, a temperature of a coolant of the engine, a temperature of the surroundings, an accumulated amount of fuel consumption from the engine start or the like may be used.
The target value may be put back to a normal value when the temperature of the HC absorbent catalyst is heated from the predetermined temperature range to an activated temperature range or when an output of the sensor disposed downstream of the HC absorbent catalyst is changed from rich to lean.
The present invention may be adaptive with a control of downstream side air-fuel ratio. For example, in the case that the first catalyst is the HC absorbent catalyst, the downstream side air-fuel ratio may be maintained at a leaner value to ensure that the air-fuel ratio in the HC absorbent catalyst is lean. In the case that the second catalyst is the HC absorbent catalyst, the downstream side air-fuel ratio may be maintained at a leaner value to ensure that the air-fuel ratio in the HC absorbent catalyst is lean.
The leaner value of the target value may be determined to maintain emissions of nitrogen oxides. In the case that the second catalyst disposed downstream is the HC absorbent catalyst, by correcting the second target value to the leaner value, it is possible to maintain emissions of nitrogen oxides.
Additionally, a deterioration of the HC absorbent catalyst may be detected based on an output of a sensor disposed on a downstream side of the HC absorbent catalyst when the target value is the leaner value. The output of the sensor disposed on a downstream side of the HC absorbent catalyst may be turned into a lean value earlier than a normal condition when the HC absorbent catalyst is deteriorated, since a desorbed amount of HC from the HC absorbent catalyst is decreased when the HC absorbent catalyst is deteriorated. Therefore, it is possible to detect the deterioration of the HC absorbent catalyst by monitoring the output of the sensor. Both of the outputs from the sensors disposed on upstream and downstream sides of the HC absorbent catalyst may be monitored to detect the deterioration more accurately.